Wood type golf club heads, for which has been mainly used persimmon, have recently turned to be those mainly made of metallic materials such as carbon steel, stainless steel, duralumin, titanium or the like. Such golf club heads can be provided with a larger head volume and a wider hitting face area as well as a larger moment of inertia for stabilizing the direction of a golf ball. In addition, a larger sweet spot in the head is obtainable so as to reduce the lowering of the resiliency of the ball on a miss hit. Moreover, a larger sized head of a golf club brings about a better stability upon address and permits a longer shaft to be fitted thereon for obtaining an increased carry of the ball.
Japanese Patent No. 2130519 (No. 33071 of Japan Patent Official Gazette of 1993) discloses a golf club head permitting an increase in carry by means of increasing the resiliency performance between the head and golf ball to the fullest. In said patent, a theory is disclosed that by means of approaching a frequency indicating the primary minimum of the mechanical impedance of the head of the golf club (hereinafter may be referred to in short as “a primary frequency of the mechanical impedance of head”,) to the frequency indicating the primary minimum of the mechanical impedance of the golf ball (hereinafter may be referred simply to as “the primary frequency of the impedance of ball” which proves to be about 600 Hz to about 1600 Hz.), the initial speed of an impacted ball is raised to the fullest (hereinafter may be referred to as “the impedance matching theory”).
“Mechanical impedance” is defined as the ratio of the magnitude of a force acting on a point to the responding velocity of another point when said force is applied. Namely, when a force applied to an object from outside and the responding velocity is expressed by F and V respectively, the mechanical impedance (Z) is defined as Z=F/V.
In order to reduce the primary frequency of the impedance of head, it is effective to reduce the rigidity of the hitting face of the head. For example, a larger area of the hitting face, a thinner hitting face, an application of a low Young's modulus material to the hitting face or the like can be cited.
In particular, it is empirically known that, the application of a low Young's modulus metallic material to the hitting face of head renders the feeling (hitting feeling) soft on hitting a golf ball and, favorable to say, even on a missed shot hitting transmits only a small shock to hands.
With a metallic material with a small tensile strength, even with a low Young's modulus, however, it is hard to secure a strength sufficient to endure a shock on impact. Moreover, to enlarge the thickness of the hitting face for obtaining the strength of the latter resulted only in little effect in reducing the rigidity of said face, which confirmed an existence of a limit also in reducing the primary frequency of the impedance of head.
Also even with a low Young's modulus, a metallic material with a small surface hardness suffers from such problems as the tendency of early wear, low scratch resistance or the like of the surface of the hitting face due to the friction with the ball on impact and sand caught between hitting face and ball on impact.